Understanding and Tuning the Electrical Conductivity of Activated Carbon: A State-of-the-Art Review

[EN] During the last decades, there has been a growing interest and research activity in the use of activated carbon (AC) and related materials as electrodes in electrochemical energy conversion and storage devices, like fuel cells, supercapacitors, and lithium-ion batteries. Among other factors, electrical properties, and especially conductivity, are well-known to play a pivotal role on the performance of ACs in these devices. Furthermore, other novel applications of AC-based materials, such as in electroadsorption, electrocatalysis, sensors and actuators, and so on, also rely heavily on their unique electrical properties. Therefore, the knowledge, understanding, and rationalization of these properties are essential with a view to assessing many of the current and future technological applications of ACs. The present paper critically reviews the available literature, including the latest published reports, on the electrical conductivity of AC. The accurate measurement of this property for ACs is rather difficult and requires the application of low to moderate compression to ensure the electrical contact. Estimated conductivity values are the result of a complex combination between a number of factors, among which the intrinsic conductivity of the single particles, their degree of contact and packing should be highlighted. Intrinsic conductivity is mainly determined by the texture, surface chemistry, and graphitization degree of AC, which strongly depend on the feedstock and the preparation method. Thus, the influence of these factors on the electrical conductivity of the resulting ACs is examined. Moreover, the influence of different adsorbed chemical species, mainly oxygen and water, is also dealt with. On the other hand, special emphasis is paid to the temperature dependence of conductivity, as its analysis is a powerful tool to gain insight into the electronic band structure and electron conduction process in carbon materials. In this regard, the different proposed mechanisms for electrical conduction in AC are exposed and compared

Catalizadores nanoestructurados basados en óxidos de cerio y libres de metal noble para la reducción electroquímica de dióxido de carbono.

Resumen de la ejecución del proyecto de líneas prioritarias "Catalizadores nanoestructurados basados en óxidos de cerio y libres de metal noble para la reducción electroquímica de dióxido de carbono" del IMEYMAT

Plant root growth against a mechanical obstacle: The early growth response of a maize root facing an axial resistance agrees with the Lockhart model

Plant root growth is dramatically reduced in compacted soils, affecting the growth of the whole plant. Through a model experiment coupling force and kinematics measurements, we probed the force-growth relationship of a primary root contacting a stiff resisting obstacle, that mimics the strongest soil impedance variation encountered by a growing root. The growth of maize roots just emerging from a corseting agarose gel and contacting a force sensor (acting as an obstacle) was monitored by time-lapse imaging simultaneously to the force. The evolution of the velocity field along the root was obtained from kinematics analysis of the root texture with a PIV derived-technique. A triangular fit was introduced to retrieve the elemental elongation rate or strain rate. A parameter-free model based on the Lockhart law quantitatively predicts how the force at the obstacle modifies several features of the growth distribution (length of the growth zone, maximal elemental elongation rate, velocity) during the first 10 minutes. These results suggest a strong similarity of the early growth responses elicited either by a directional stress (contact) or by an isotropic perturbation (hyperosmotic bath)

Preparation of activated carbon-metal (hydr) oxide materials by thermal methods. Thermogravimetric-mass spectrometric (TG-MS) analysis

[EN] tActivated carbon (AC)-metal (hydr) oxide (MO) materials prepared by wet impregnation of a commercialAC with Al3+, Fe3+, Zn2+, SnCl2, TiO2and WO42−in water at pH between 1.37 and 9.54 in two successivesoaking and oven-drying steps are analysed by thermogravimetry-mass spectrometry (TG-MS) between25 and 900◦C. Under identical conditions, a blank sample (ACB) was first prepared using deionized waterand then thermally analyzed. The mass loss in the TG analysis is 1.35 wt% for AC, 2.12 wt% for ACB, andbetween 2.47 and 23.20 wt% for the AC-MO materials. For these materials, it depends on the impregnationagent and varies by SnCl2> Fe3+> Al3+> Zn2+> WO42−> TiO2. The number of thermal effects giving off CO2and CO is larger with the Al3+, Fe3+and Zn2+ions than with SnCl2, TiO2and WO42−. It is so in particular forCO2with the Fe3+ion and for CO with the Al3+ion. In general, the release of CO at high temperature hasbeen associated with the carbothermal reduction of metal oxides. As a result of the reaction low-meltingpoint metals such as Al, Zn and Sn, unlike Fe and W, are formed which vaporize and thereby contribute tothe mass loss. Only with Fe3+and TiO2, the amount of desorbed water is larger by dehydroxylation thanby dehydration. The masses of evolved H2O, CO2and CO are by far higher with SnCl2, whereas they arelower with TiO2and WO42−

Surface morphological characterization of activated carbon-metal (hydr)oxide composites: some insights into the role of the precursor chemistry in aqueous solution

[EN] Morphological features of metal (hydr)oxide (MO) particles supported on activated carbon (AC) largely influence the performance of these composite materials in most of their applications, particularly in heterogeneous catalysis. Furthermore, the MO precursor as well as the preparation method and conditions strongly determine these morphological features. Thus, the present work is aimed at shedding light on the role of the precursor chemistry on the surface morphology of a series of AC-MO composites prepared by wet impregnation of a commercial AC with Al(NO3)3, Fe(NO3)3, and Zn(NO3)2 in aqueous solution. These materials are characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The microstructure, morphology, size distribution and degree of dispersion of the supported MO (nano)particles strongly depend on the chemical transformations undergone by the precursors not only in the impregnation solutions after their contact with AC but also during the oven-drying step. Al3þ, Fe3þ and Zn2þ species in aqueous medium are involved in hydrolysis and polymerization processes, which notably modify the pH of the starting precursor solutions. Upon their contact with AC, pH markedly increases due to the strong basic character of the carbon surface (pHpzc 10.50), leading to the precipitation of the metal hydroxides or oxyhydroxides. Both supported bayerite (a-Al(OH)3) and goethite (a-FeO(OH)) are essentially amorphous; however, the former grows in micrometric particles while the latter does as nanoparticles. By contrast, the higher crystallinity and larger particle size of supported w€ulfingite (e-Zn(OH)2) are connected with an additional transformation of the as-precipitated amorphous hydroxide during the heating step at 120 C

Physico-chemical characterization of activated carbon–metal oxide photocatalysts by immersion calorimetry in benzene and water

[EN] From a commercial activated carbon (AC) and Al3?, Fe3?, Zn2?, SnCl2, TiO2 and WO4 2- in water, three series of AC–metal (hydr)oxide (MO) samples prepared by wet impregnation in two successive steps of soaking at 80 ºC and oven-drying at 120 ºC (S1) and subsequent heat treatment at 200 (S2) or 850 C (S3) were characterized texturally by N2 adsorption at -196 C and by immersion calorimetry in benzene and water. The mass changes associated with the preparation of the samples are usually stronger for S1 and S3 than for S2. The incorporation of MO to AC causes a greater decrease in the micropore volume and pore narrowing only for the SnCl2-impregnated sample. The opposite effects on the microporous structure are noted for most S2 and S3, as compared to S1. For AC, -DiH(C6H6) is 114.0 J g-1 and -DiH(H2O) is 30.5 J g-1. For the AC–MO samples, -DiH(C6H6) and Stot(C6H6) are generally lower than for AC and vary by S3[S2[S1. However, -DiH(H2O), Stot(H2O) and [O] are usually lower for S3. Stot(C6H6) is higher for the samples prepared using the metal ions. The results of immersion calorimetry for AC and AC–MO samples provide one with valuable information concerning the dependence of the hydrophobicity and hydrophilicity of the samples on the method used in their preparation

Preparation and Microstructural Characterization of Activated Carbon-Metal Oxide Hybrid Catalysts: New Insights into Reaction Paths

[EN] In catalysis processes, activated carbon (AC) and metal oxides (MOs) are widely used either as catalysts or as catalyst supports because of their unique properties. A combination of AC and MO nanoparticles in a single hybrid material usually entails both chemical and microstructural changes, which may largely influence the potential catalytic suitability and performance of the resulting product. Here, the preparation of a wide series of AC MO hybrid catalysts is studied. Three series of such catalysts are prepared by support first of MO (Al2O3, Fe2O3, SnO2, TiO2, WO3, and ZnO) precursors on a granular AC by wet impregnation and oven-drying at 120 ºC, and by subsequent heat treatment at 200 or 850 ºC in inert atmosphere. Both the chemical composition and microstructure are mainly investigated by powder X-ray diffraction. Yield and ash content are often strongly dependent on the MO precursor and heat treatment temperature, in particular for the Sn catalysts. With the temperature rise, trends are towards the transformation of metal hydroxides into metal oxides, crystallinity improvement, and occurrence of drastic composition changes, ultimately leading to the formation of metals in elemental state and even metal carbides. Reaction paths during the preparation are explored for various hybrid catalysts and new insights into them are provided

Particle size distribution and morphological changes in activated carbon‐metal oxide hybrid catalysts prepared under different heating conditions

[EN] In catalysis processes, activated carbon (AC) andmetal oxides (MOs) are widely used either as catalysts or as catalyst supports because of their unique properties. A combination of AC and a MO in a single hybrid material entails changes not only in the composition, microstructure and texture but also in the morphology, whichmay largely influence the catalytic behaviour of the resulting product. Thiswork isaimedat investigating the modifications in themorphology and particle size distribution (PSD) for AC-MO hybrid catalysts as a result of their preparation under markedly different heating conditions. From a commercial AC and six MO (Al2O3, Fe2O3, ZnO, SnO2, TiO2 andWO3) precursors, two series of such catalysts are prepared by wet impregnation, oven-drying at 120ºC, and subsequent heat treatment at 200ºC or 850ºC in inert atmosphere. The resulting samples are characterized in terms of theirmorphology andPSDby scanning electronmicroscopy and ImageJ processing program. Obtained results indicate that the morphology, PSD and degree of dispersion of the supported catalysts are strongly dependent both on the MO precursor and the heat treatment temperature. With the temperature rise, trends are towards the improvement of crystallinity, the broadening of the PSD and the increase in the average particle size, thus suggesting the involvement of sinteringmechanisms. Such effects aremore pronounced for the Fe, Sn andWcatalysts due to the reduction of the corresponding MOs by AC during the heat treatment at 850ºC

FT-IR Analysis of Pyrone and Chromene Structures in Activated Carbon

[EN] The behavior of activated carbon in many catalysis processes is determined by the basic character of its surface. Using FT-IR spectroscopy, an attempt is made here to identify not only pyrone and chromene type structures but also their isomers in a commercial activated carbon (Merck; AC). The infrared analysis between 400 and 4000 cm−1 focuses on the keto (−CO) group in pyrones, methylene (−CH2−) group in chromenes, and enol ether group (O−CC) in both kinds of structures. Although overlapping bands are frequent in the AC spectrum, occurrence of most significant pyrone and chromene absorptions proves their presence in AC. This is so in particular for 2-pyrone and 4H-chromene, whereas 4-pyrone is less amenable to the infrared analysis

Preparation of activated carbon-metal oxide hybrid catalysts: textural characterization

[EN] In catalysis processes, activated carbon (AC) and metal oxides (MOs) are used as catalysts and catalyst supports because of their textural and chemical properties. A combination of AC andMO properties in a single catalyst entails changes in the catalytic activity and behaviour which would redound to the number of applications. The present study aims at preparing AC-MO hybrid catalysts by chemical interaction of MO precursors in aqueous medium with AC and at carrying out the textural characterization of the samples. From a commercial AC and six MO precursors (i.e. Fe3+, Al3+, Zn2+, SnCl2, TiO2, andWO4 2 −), three series of hybrid catalysts were prepared by wet impregnation and oven-drying at 120 °C and subsequent heat treatment of the resulting products at 200 or 850 °C in inert atmosphere. The samples were characterized texturally by N2 adsorption at−196 °C, mercury porosimetry, and density measurements. Therefore, the influence of theMO precursor and heating conditions on the porous texture is studied. Yield varies more widely for the samples prepared at 120 °C and 850 °C than at 200 °C. The mass increase after oven-drying at 120 °C and the mass decrease after heating at 850 °C are much greater for the Sn catalysts. Because of the support ofMO precursors on AC, in general, macro-, meso-, and microporosity significantly decrease. The effects on the texture of AC are by far more important for the Sn catalyst and also, though less, for the Fe catalyst. However, they are weaker for theW and Ti catalysts. In general, the heat treatment at 200 °C only causes small changes in the porous texture of the samples. By heating at 850 °C the pore size distribution becomes more uniform in the three porosity regions. Microporosity develops chiefly for the Sn catalyst, whereas mesoporosity does mainly for the Sn and Fe catalysts. The textural modifications have been associated with mass, composition, and structural modifications